Usually the hot forming is performed by providing a blank, heating the blank to a temperature between 700° and 1000° C. and keeping the blank at that temperature for a few minutes, placing the heated blank in a hot forming apparatus, forming the blank into a part in the hot forming apparatus, and hardening the hot formed part.
When using uncoated steel, the heating of the blank before the hot forming in a hot forming apparatus can be performed under a protective atmosphere to prevent oxidation and decarburization of the steel. However, the hot forming itself and the hardening take place in air, and thus oxidation occurs; therefore, after the hot forming the hot formed parts must be descaled. To overcome this drawback, in the last ten years it has been proposed to use coated steel sheets, which sheets are heated to a temperature above the Ac1 temperature. During the heating a diffusion layer is formed between the coating and the steel sheet, providing protection against oxidation and a good adherence of the coating to the steel sheet, also at the elevated temperatures which are used for hot forming.
Though a protective atmosphere is not necessary anymore when using coated steel sheets, the known method has some drawbacks. One of the main problems is that the heating velocity of the coated steel sheets has been found to be critical. This makes the whole process more difficult to control. It also results in the heating of a steel sheet taking a considerable time, for instance 5 minutes, whereas the hot forming in the hot forming apparatus and the subsequent hardening can be performed in less than 1 minute. Manufacturing at a high production rate, as made possible by the hot forming apparatus, can be performed by heating a number of coated steel sheets in an oven. However, when there is a delay at the hot forming apparatus the coated steel sheets remain too long in the oven, which means that they have to be scrapped. This has a considerable influence on the cost of the hot forming process. Moreover, the oven has to be very long.